Heterosexual relations currently constitute the predominant route of HIV transmission to women in Brazil. Few studies have approached male sexual behavior taking women into account. This study included 597 men ages 21 to 50 in São Paulo city and explored several aspects pertaining to safer sex, such as sexual partners, knowledge of sexually transmitted diseases (STDs)/AIDS, risk perception, and protective practices aimed at AIDS prevention. The results showed that, similar to other countries, the proportion of men who have sex with men is low (5%), that younger men adopt more protective behavior than older men regarding multiple partners and condom use, that knowledge of AIDS is widespread (but that there is still a lack of knowledge concerning STDs in general), and that although risk perception increases protective behavior, presumption of the partner's behavior influences this perception, placing men and women in a vulnerable situation.